.TH std::uses_allocator_construction_args 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::uses_allocator_construction_args \- std::uses_allocator_construction_args

.SH Synopsis
   Defined in header <memory>
   T is not a specialization of std::pair
   template< class T, class Alloc, class... Args >

   constexpr auto uses_allocator_construction_args( const Alloc&      \fB(1)\fP \fI(since C++20)\fP
   alloc,

       Args&&... args ) noexcept;
   T is a specialization of std::pair
   template< class T, class Alloc, class Tuple1, class Tuple2 >

   constexpr auto uses_allocator_construction_args( const Alloc&      \fB(2)\fP \fI(since C++20)\fP
   alloc,

       std::piecewise_construct_t, Tuple1&& x, Tuple2&& y ) noexcept;
   template< class T, class Alloc >
   constexpr auto uses_allocator_construction_args( const Alloc&      \fB(3)\fP \fI(since C++20)\fP
   alloc ) noexcept;
   template< class T, class Alloc, class U, class V >

   constexpr auto uses_allocator_construction_args( const Alloc&      \fB(4)\fP \fI(since C++20)\fP
   alloc,

       U&& u, V&& v ) noexcept;
   template< class T, class Alloc, class U, class V >

   constexpr auto uses_allocator_construction_args( const Alloc&      \fB(5)\fP (since C++23)
   alloc,

       std::pair<U, V>& pr ) noexcept;
   template< class T, class Alloc, class U, class V >

   constexpr auto uses_allocator_construction_args( const Alloc&      \fB(6)\fP \fI(since C++20)\fP
   alloc,

       const std::pair<U, V>& pr ) noexcept;
   template< class T, class Alloc, class U, class V >

   constexpr auto uses_allocator_construction_args( const Alloc&      \fB(7)\fP \fI(since C++20)\fP
   alloc,

       std::pair<U, V>&& pr ) noexcept;
   template< class T, class Alloc, class U, class V >

   constexpr auto uses_allocator_construction_args( const Alloc&      \fB(8)\fP (since C++23)
   alloc,

       const std::pair<U, V>&& pr ) noexcept;
   template< class T, class Alloc, class NonPair >

   constexpr auto uses_allocator_construction_args( const Alloc&      \fB(9)\fP \fI(since C++20)\fP
   alloc,

       NonPair&& non_pair ) noexcept;

   Prepares the argument list needed to create an object of the given type T by means
   of uses-allocator construction.

   1) This overload participates in overload resolution only if T is not a
   specialization of std::pair. Returns std::tuple determined as follows:
     * If std::uses_allocator_v<T, Alloc> is false and std::is_constructible_v<T,
       Args...> is true, returns std::forward_as_tuple(std::forward<Args>(args)...).
     * Otherwise, if std::uses_allocator_v<T, Alloc> is true and
       std::is_constructible_v<T, std::allocator_arg_t, const Alloc&, Args...> is true,
       returns
       std::tuple<std::allocator_arg_t, const Alloc&, Args&&...>(std::allocator_arg,
       alloc,

       std::forward<Args>(args)...).
     * Otherwise, if std::uses_allocator_v<T, Alloc> is true and
       std::is_constructible_v<T, Args..., const Alloc&> is true, returns
       std::forward_as_tuple(std::forward<Args>(args)..., alloc).
     * Otherwise, the program is ill-formed.
   2) This overload participates in overload resolution only if T is a specialization
   of std::pair. For T that is std::pair<T1, T2>, equivalent to

 return std::make_tuple(std::piecewise_construct,
     std::apply([&alloc](auto&&... args1)
         {
             return std::uses_allocator_construction_args<T1>(alloc,
                        std::forward<decltype(args1)>(args1)...);
         }, std::forward<Tuple1>(x)
     ),
     std::apply([&alloc](auto&&... args2)
         {
             return std::uses_allocator_construction_args<T2>(alloc,
                        std::forward<decltype(args2)>(args2)...);
         }, std::forward<Tuple2>(y)
     )
 );

   3) This overload participates in overload resolution only if T is a specialization
   of std::pair. Equivalent to

 return std::uses_allocator_construction_args<T>(alloc,
     std::piecewise_construct, std::tuple<>{}, std::tuple<>{}
 );

   4) This overload participates in overload resolution only if T is a specialization
   of std::pair. Equivalent to

 return std::uses_allocator_construction_args<T>(alloc,
     std::piecewise_construct,
     std::forward_as_tuple(std::forward<U>(u)),
     std::forward_as_tuple(std::forward<V>(v))
 );

   5,6) This overload participates in overload resolution only if T is a specialization
   of std::pair. Equivalent to

 return std::uses_allocator_construction_args<T>(alloc,
     std::piecewise_construct,
     std::forward_as_tuple(pr.first),
     std::forward_as_tuple(pr.second)
 );

   7,8) This overload participates in overload resolution only if T is a specialization
   of std::pair. Equivalent to

 return std::uses_allocator_construction_args<T>(alloc,
     std::piecewise_construct,
     std::forward_as_tuple(std::get<0>(std::move(pr))),
     std::forward_as_tuple(std::get<1>(std::move(pr)))
 );

   9) This overload participates in overload resolution only if T is a specialization
   of std::pair, and given the exposition-only function template

 template<class A, class B>
 void /*deduce-as-pair*/(const std::pair<A, B>&);

   , /*deduce-as-pair*/(non_pair) is ill-formed when considered as an unevaluated
   operand.
   Let the exposition-only class pair-constructor be defined as

 class /*pair-constructor*/
 {
     const Alloc& alloc_; // exposition only
     NonPair&     u_;     // exposition only

     constexpr reconstruct(const std::remove_cv<T>& p) const // exposition only
     {
         return std::make_obj_using_allocator<std::remove_cv<T>>(alloc_, p);
     }

     constexpr reconstruct(std::remove_cv<T>&& p) const // exposition only
     {
         return std::make_obj_using_allocator<std::remove_cv<T>>(alloc_, std::move(p));
     }

 public:
     constexpr operator std::remove_cv<T>() const
     {
         return reconstruct(std::forward<NonPair>(u_));
     }
 };

   This overload is equivalent to return std::make_tuple(pair_construction);, where
   pair_construction is a value of type pair-constructor whose alloc_ and u_ members
   are alloc and non_pair respectively.

.SH Parameters

   alloc    - the allocator to use
   args     - the arguments to pass to T's constructor
   x        - tuple of arguments to pass to the constructors of T's first data member
   y        - tuple of arguments to pass to the constructors of T's second data member
   u        - single argument to pass to the constructor of T's first data member
   v        - single argument to pass to the constructor of T's second data member
              a pair whose first data member will be passed to the constructor of T's
   pr       - first data member and second data member will be passed to the
              constructor of T's second data member
   non_pair - single argument to convert to a std::pair for further construction

.SH Return value

   std::tuple of arguments suitable for passing to the constructor of T.

.SH Notes

   The overloads (2-9) provide allocator propagation into std::pair, which supports
   neither leading-allocator nor trailing-allocator calling conventions (unlike, e.g.
   std::tuple, which uses leading-allocator convention).

   When used in uses-allocator construction, the conversion function of
   pair-constructor converts the provided argument to std::pair at first, and then
   constructs the result from that std::pair by uses-allocator construction.

.SH Example

    This section is incomplete
    Reason: no example

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to          Behavior as published              Correct behavior
   LWG 3525 C++20      no overload could handle non-pair types  reconstructing overload
                       convertible to pair                      added

.SH See also

   uses_allocator                          checks if the specified type supports
   \fI(C++11)\fP                                 uses-allocator construction
                                           \fI(class template)\fP
   make_obj_using_allocator                creates an object of the given type by means
   (C++20)                                 of uses-allocator construction
                                           \fI(function template)\fP
                                           creates an object of the given type at
   uninitialized_construct_using_allocator specified memory location by means of
   (C++20)                                 uses-allocator construction
                                           \fI(function template)\fP

.SH Category:
     * Todo no example
